In order to make the solar thermal energy more affordable, in addition to improvement in the solar thermal power plant design and associated support structure for solar fields, solar absorber coatings with improved optical properties and thermal stability need to be developed. An ideal high-temperature solar selective coating must have high absorptance (>0.940), low thermal emittance (≦0.10 at 400° C.), stability up to 500° C. in air with improved durability and manufacturability and reduced cost [reference may be made to C. E. Kennedy, Proceedings of International Solar Energy Conference, Aug. 6-12, 2005, Orlando, USA]. A variety of physical vapor deposition processes—PVD—(such as evaporation, ion plating, cathodic arc evaporation, pulsed laser deposition and sputtering) have been used to develop high-temperature solar selective coatings. Pt—Al2O3, Ni—Al2O3, Ni—SiO2, Cr—SiO, Mo—Al2O3, Mo—SiO2, W—Al2O3 coatings have been used for the high temperature solar selective applications. Although these coatings have good thermal stability in vacuum and low thermal stability (≦300° C.) in air, suitable coating modifications have been done to improve the thermal stability of these coatings up to some extent.
One of the most essential requirements of solar selective absorbers is their stable structural composition when they operate at high temperatures. Optical properties of these coatings should not degrade with rise in temperature or over a period of use.
The hybrid multilayer solar selective coating of the present invention is deposited on metallic and non-metallic substrates preferably on stainless substrates (SS 304 and 321) with and without chrome plating using sputtering process in combination with sol-gel method, which are environmental friendly. The main utility of the present invention is for high temperature applications, particularly, in solar steam generators and steam turbines for producing electricity.
Earlier, the applicant had developed a high temperature thermally stable solar selective coating for effectively harnessing the solar energy. Patent application is filed in India (Application No. 3655DEL2011). In this patent, a multilayer solar selective coating containing tandem stacks of Ti/chrome interlayer, aluminum titanium nitride, aluminum titanium oxynitride, aluminum titanium oxide was coated on metallic and non-metallic substrates using a sputtering method. This coating exhibits absorptance of 0.930 and emittance of 0.160.17 on stainless steel substrates and displays thermal stability in air up to 350° C. and in vacuum up to 450° C. for longer durations under cyclic heating conditions. The coating also displays improved adhesion, UV stability, corrosion resistance and stability under extreme environments. But this invention has two limitations: (i) the absorptance is 0.930 and thermal stability is low (350° C. in air 450° C. in vacuum). These limitations of the earlier invention directed the inventors to evolve a hybrid multilayer solar selective coating using a novel combination of sputtering and sol-gel methods for depositing a hybrid multilayer solar selective coating suitable for high temperature applications for effectively harnessing solar energy.
In the present invention, an organically modified silica (ormosil) layer is deposited over the multilayer coating containing tandem stacks of Ti/chrome interlayer, aluminum titanium nitride (AlTiN), aluminum titanium oxynitride (AlTiON), aluminum titanium oxide (AlTiO) on metallic and non-metallic substrates more preferably on stainless steel 304 and 321 substrates. The chrome interlayer was prepared using a standard electroplating process, whereas, Ti, AlTiN, AlTiON and AlTiO layers were prepared using a four-cathode reactive unbalanced pulsed direct current magnetron sputtering technique. The ormosil layer, deposited using a sol-gel/dip coating technique, provides the enhanced absorptance and improved high thermal stability in air and vacuum for the hybrid multilayer solar selective coating of the present invention.
The present invention provides a hybrid multilayer solar selective coating having absorptance >0.950, emittance <0.11 (on chrome plated SS substrate) and high thermal stability (long life in the order of 1000 hrs under cyclic heating conditions in air at 500° C.). It also provides a hybrid multilayer solar selective coating having higher thermal stability in vacuum at 600° C. up to 1000 hrs under cyclic heating conditions. Hybrid multilayer solar selective coating of the present invention exhibits higher solar selectivity ratio in the order of 5-9 on metal and non-metal substrates. The hybrid multilayer solar selective absorber coating of the present invention has high oxidation resistance, stable microstructure, high adherence and graded composition particularly useful for high temperature solar thermal power generations.
Prior-art search was made in public domain for patent as well as non-patent literature to find out the related work carried out, in areas of the present invention. Some of the recent works, which are related to the field of the present invention, are discussed below.
In order to improve the overall solar selectivity of the absorber coatings, it is necessary to combine two or more deposition techniques. For example, a suitable protective coating with anti-reflective property prepared by sol-gel method can be applied on standard PVD absorber coatings. As will be discussed in the prior-art, such attempts have been made in the past for electrodeposited black chrome coating. For example, sol-gel protective coatings have been used for black chrome solar selective films [reference may be made to R. B. Pettit and C. J. Bruker, SPIE Optical Coatings for Energy Efficiency and Solar Applications 324 (1982) 176]. The sol-gel coating process consists of applying an alcoholic solution containing polymeric glass precursors. After the coating is fixed for one-half hour at 450° C., a glass layer was obtained. For the best combination of process variables, the solar absorptance of the sol-gel coated sample decreased from 0.97 to 0.95 after 100 hrs at 400° C., while for an uncoated black chrome coating, the absorptance decreased to 0.89. At 400° C., the sol-gel protected the black chrome coating.
The use of sol-gel thin films in solar energy applications has been reviewed by Pettit and Brinker [Solar Energy Materials 14 (1986) 269]. Sol-gel thin films for solar energy applications have been used in: (i) encapsulation of black chrome solar selective coatings, for improved thermal stability (ii) formation of porous, antireflection coating on glass envelopes used in solar thermal collectors for increased transmittance, (iii) double-layer, antireflection coatings of SiO2 and TiO2 on silicon solar cells to improve cell efficiency and (iv) protective coatings applied to silvered stainless steel solar mirrors.
References may be made to U.S. Pat. No. 6,783,653 B2 wherein a sol-gel layer protects the structured metallic overlayer of the solar selective absorption coatings. The selective absorber layer contains pinnacles which are tall and thin and have dimensions and inter-pinnacle spacing such that the absorptivity of the coating is very high in the solar spectrum while the emissivity in the infrared region is very low. The absorber layer is protected both physically and chemically by a sol-gel layer, which is comprised of a network of highly polymerized monomers. The monomers are typically oxides of network forming elements (e.g., Al, B, Mg, Ti, Si, Zn, etc.). It is claimed that the sol-gel layer provides mechanical stability and environmental protection to about 350° C. and also enhances the solar absorption of the coating. This invention uses copper as the substrate, which cannot be used for high temperature applications. Further the Sol gel layer provides mechanical stability and environmental protection only up to 350° C.
Reference may be made to Harizanov et al., Ceramics International 22 (1996) 91, wherein the sol-gel and chemical vapor deposition (CVD) methods have been used independently to prepare oxide coatings for solar energy utilization. The sol-gel coating consisted of TiO2/0.25 MnO, while the CVD coating consisted of WO3. It has been shown that the sol-gel coating offers promising performance in passive solar control glazing due to the relatively high refractive index. This paper doesn′t disclose the information on the absorptance, emittance and solar selectivity of the sol gel and CVD metal oxide coatings, which decides the application of coatings for solar thermal power generation.
References may be made to US patent No. US 2011/0003142 A1, wherein nanoparticle composite hybrid transparent coating has been prepared by sol-gel process. The composite hybrid thick transparent hard coating is the gelled dispersion of nanoparticles in a sol with at least one hydrolysable silane and at least one hydrolysable metal oxide precursor. The invention reports deposition of 5 μm thick hybrid coating even on plastic substrates. This invention discloses only mechanical properties of 5 μm thick hybrid transparent sol-gel coating and there is no mention of optical properties of coating.
References may be made to Katumba et al., Solar Energy Materials and Solar Cells 92 (2008) 1285, wherein carbon nanoparticles embedded in ZnO and NiO selective solar absorbers have been prepared using sol-gel technique. ZnO based absorber coatings exhibited thermal emittance, of 6% and absorptance of 71%, whereas NiO based samples exhibited thermal emittance of 4% and absorptance of 84%. The substrate used in this work is suitable for low temperature application and not for high temperature application those are needed in case of solar thermal power generation.
References may also be made to Vince et al., Solar Energy Materials and Solar Cells 79 (2003) 313, wherein CoCuMnOx absorber coatings have been prepared by sol-gel process. These coatings exhibited absorptance of 0.85-0.91 and emittance of 0.036 on Al substrates. The researchers have tested the coating deposited on Aluminum in the boiling water (−100° C.) and no high temperature test have been reported in the paper. It should be noted that Aluminum is suitable for low temperature application.
References may also be made to Lira-Cantx et al., Solar Energy Materials and Solar Cells 87 (2005) 685, wherein the use of silica based sol-gel anti-reflection coating on black nickel solar absorber coating has been reported. The silica coating improved the absorptance when drying at 200° C., however, the absorptance decreased while heating the coating system at 300° C. due to degradation of the black nickel surface. The thickness of the silica coating was approximately 400 nm. The absorber coating developed by the researchers was prepared by the electro-chemical route and thus it is not suitable for high temperature application.
References may be made to Nostell et al., Thin Solid Films 351 (1999) 170, wherein silica based anti-reflective films on glass have been prepared by a dip-coating method from a sol-gel process. The total solar transmittance of silica anti-reflective coating increased by 5.4%. The coatings also exhibited improved scratch resistance and the adhesion between the film and the substrate after baking the silica film at higher temperatures. Similar improvement in the transmittance of a transparent cover by depositing anti-reflective transparent sol-gel coating has been reported by Gombert et al. [Solar Energy 68 (2000) 357]. The coatings presented in these works have been developed on glass substrate for improving the anti reflection properties. The papers doesn′t provide any information on absorptance and emittance of the coating.
In order to simplify the solar energy system design and lower the cost of solar thermal power plant, there is a universal requirement for the development of solar selective coatings, which can operate at very high temperatures in air. A considerable technology development needs to be carried out in this direction, because prolonged heating of solar absorber coatings in air at higher temperatures can not only oxidize the coating, but also can induce other microstructural changes which degrade the overall solar selectivity of the spectrally selective coatings. Manufacturing a hybrid coating using two or more methods can play a significant role in evolving such a coating. The sputtering method provides the basic solar absorber coating with an absorptance of 0.930 and emittance of 0.16-0.17 on SS substrates. The ormosil layer deposited by sol-gel dip coating provides enhanced absorptance (>0.950) and improved thermal stability (−500° C. in air). Therefore, hybrid coatings consisting of PVD and other processes (e.g., sol-gel) need to be developed in order to reduce the cost of solar absorber coating technology. Prior-art referred as above shows the only combination of sol-gel with black chrome coating and CVD. But these methods provide a coating for mid-temperature applications and these coatings are reported to have thermal stability only up to 400° C. for shorter durations. Also inorganic sol-gel coatings are susceptible to cracking, thus limiting their applications for improving the thermal stability of the solar absorber coatings for high temperature applications. None of the prior-art shows the combination of sputtering and sol-gel coating for designing a high temperature solar selective coating with thermal stability up to 500° C. in air. In the present invention, two deposition routes (sputtering and sol-gel) have been used for designing the improved high temperature solar selective coating. Interlayer is deposited on substrate before depositing absorber layers for reducing the emittance and the barrier layer is deposited for improving the thermal stability.